Optical parametric Oscillators (OPO) are passive, optical devices which convert a high intensity laser beam (pump) into two beams (signal and idler) having different wavelengths than the pump beam. In this technique, a laser beam is directed to pass through a non-linear optical crystal such as BaBa.sub.2 O.sub.4, LiB.sub.3 O.sub.5, LiNbO.sub.3, KTiOPO.sub.4, or the like. The interaction between the high intensity electrical field of the laser pump beam and the crystal results in conversion of the pump beam into two beams of different wavelengths (signal and idler) while preserving momentum and energy. The wavelengths of these beams can be tuned over a wide spectral range by varying the orientation of the crystal with respect to the laser beam, by changing the crystal's temperature, or by applying a variable voltage across the crystal. Various tuning ranges can be achieved by properly selecting the laser, the non-linear crystal and the optical components of the system.
Different constraints limit the practicality of presently available OPO designs. The high laser intensity required from the laser source to efficiently pump an OPO, can exceed the optical damage threshold of the non-linear crystal, or of the dielectric coatings of the various optical components within the cavity.
In linear cavities of the prior art, the pump beam traverses the cavity in one direction whereas the signal and the idler beams oscillate back and forth. However, the parametric process takes place only when the parametric beams (signal and idler) propagate in the same direction as the laser pump beam. This severely limits the efficiency of presently available systems. Another limitation with presently available systems is presented by the walkoff associated with the propagation of extraordinary beams in birefringent materials, such as the non-linear crystal used in the OPO. The walkoff can limit the effective gain length of the crystal, and therefore the conversion efficiency.
In critical phase-matching configurations, the beams which are generated by the non-linear process propagate with different polarization than the laser beam. The parametric gain length is defined as the length within the crystal along which the beams interact to stimulate gain. The walkoff that is associated with the propagation of beams with different polarizations within a birefringence material limits the effective gain length. The walkoff angle depends on the choice of non-linear crystal, the pump wavelength, and the geometric configuration (e.g., in Type I OPO in BBO this angle is in the range of 2-5 deg, and the effective gain length is limited to a few diameters of the pump beam).
A technique to increase the gain length by reducing the walkoff is disxclosed in U.S. Pat. Nos. 5,047,668 and 5,079,445. The walkoff compensation presented in these patents is achieved by placing two crystals with their optical axes opposing each other. However, the two crystals have to be rotated in a counter direction to each other with great mechanical precision in order to allow tuning.
Finally, the system efficiency is significantly impaired due to the Fresnel losses inside the cavity. Since the idler and the signal are tuned continuously over a wide spectral range, it is practically impossible to provide an Anti-Reflection (AR) coating that will reduce the surface reflection losses over a spectral range that will cover the pump, signal, and idler wavelengths.
These problems have to be resolved in order to build tunable laser systems which will be reliable and efficient, be easily tunable and adaptable to various uses. Therefore, there is a need for an OPO system that offers solutions to these problems and which can be operated with commercially available lasers, yet can be tuned over a wide spectral range without risking optical damage. Such a system should also be small and simple to align and operate. Such a system will close the gap between conceptual ideas and a useful and practical apparatus for research, medical applications and industrial applications.